Transcript Concave and Convex Lenses

19.2 Lenses
Light rays slow and
bend as they pass
through the curved
glass lens. In this
case, the result is a
magnified image.
19.2 Lenses
Index of Refraction of Light
What causes light to refract?
When light enters a new medium at an angle,
the change in speed causes the light to
bend, or refract.
19.2 Lenses
Index of Refraction of Light
Light usually travels in straight lines. In a
vacuum, light travels at a speed of 3.00 × 108
meters per second.
• The speed of light in a different medium
depends on the material of the new medium.
• Air allows light to pass through almost as fast
as it would through a vacuum.
19.2 Lenses
Index of Refraction of Light
The speed of light in water is 2.25 × 108
meters per second. The speed of light in
glass is 2.00 × 108 meters per second.
• When light passes from air into glass or water,
it slows down.
• When light passes from glass or water into air,
it speeds up.
19.2 Lenses
Index of Refraction of Light
A light ray bends (refracts)
as it passes through media
with different indices of
refraction.
The ray again travels in its
original direction when it
reenters the air.
19.2 Lenses
Index of Refraction of Light
The amount of refraction depends upon the
difference between the speeds of light in the
two media. The index of refraction for a
material is the ratio of the speed of light in a
vacuum to the speed of the light in the
material.
• A low index of refraction (near 1) causes light to
slow and refract very little.
• Diamond (index of refraction = 2.42), causes
significant refraction.
19.2 Lenses
Properties of Gemstones
Gemstones used in jewelry are known for
several of their physical properties—primarily
luster and optical brilliance. Luster is a
measure of the amount of light that strikes a
gemstone’s surface and is reflected. Flat and
smooth surfaces increase a gemstone’s
luster.
19.2 Lenses
Properties of Gemstones
Like luster, the brilliance of a gemstone involves
reflected light. Light that is not reflected by a gem’s
lustrous surface passes into the stone. The
brilliance of a gemstone is a measure of the
amount of light entering the gem that is reflected
back to the viewer.
Precise techniques are used to cut gemstones into
shapes that produce maximum brilliance. The
combination of a specialized shape and the
gemstone’s inherent high index of refraction gives
gems their brilliance.
19.2 Lenses
Properties of Gemstones
The table summarizes the index of refraction and
luster of several common gemstones. Note that
moissanite is a manufactured material used to
simulate diamond.
19.2 Lenses
Properties of Gemstones
1. Interpreting Tables Which material is the most lustrous?
The least lustrous?
19.2 Lenses
Properties of Gemstones
1. Interpreting Tables Which material is the most lustrous?
The least lustrous?
Answer: Moissanite is the most lustrous (20.4%); emerald is
the least lustrous (4.8%).
19.2 Lenses
Properties of Gemstones
2. Calculating What percentage of light striking a sapphire
gemstone enters it?
19.2 Lenses
Properties of Gemstones
2. Calculating What percentage of light striking a sapphire
gemstone enters it?
Answer: 96.2% of the light striking a sapphire enters the
gemstone.
19.2 Lenses
Properties of Gemstones
3. Applying Concepts If a light ray strikes each material at
an angle, in which material would the light ray bend the
most?
19.2 Lenses
Properties of Gemstones
3. Applying Concepts If a light ray strikes each material at
an angle, in which material would the light ray bend the
most?
Answer: Light bends the most in moissanite because it has
the greatest index of refraction.
19.2 Lenses
Properties of Gemstones
4. Applying Concepts The speed of light through an unknown gemstone
is 1.69 × 108 m/s. Identify the gemstone.
19.2 Lenses
Properties of Gemstones
4. Applying Concepts The speed of light through an unknown gemstone
is 1.69 × 108 m/s. Identify the gemstone.
Answer: Index of refraction = speed of light in vacuum/speed of light in
gemstone; index of refraction = (3.00 × 108 m/s)/ (1.69 × 108 m/s) =
1.77; the gemstone could be either a ruby or a sapphire.
19.2 Lenses
Concave and Convex Lenses
What type of images do concave and convex
lenses form?
Concave lenses always cause light rays to
spread out and can only form virtual images.
Convex lenses form either real or virtual
images.
19.2 Lenses
Concave and Convex Lenses
Concave Lenses
A lens is an object made of transparent material
that has one or two curved surfaces that can
refract light.
A concave lens is curved inward at the center
and is thickest at the outside edges.
19.2 Lenses
Concave and Convex Lenses
The rays enter the lens at different angles, and
so they emerge from the lens at different angles.
• Concave lenses cause incoming parallel rays to
spread out, or diverge. The diverging rays appear
to come from the focal point, on the same side of
the lens as the object.
• A concave lens forms a virtual image. The image
formed by a concave lens is always smaller than
the object.
19.2 Lenses
Concave and Convex Lenses
A. When parallel incoming rays strike a concave
lens, they are refracted away from one another.
Optical axis
Focal point
Concave lens
19.2 Lenses
Concave and Convex Lenses
B. As the light rays diverge after passing through
the concave lens, they form a virtual image of
the object.
Object
Focal point
Virtual
image
Concave lens
Focal point
19.2 Lenses
Concave and Convex Lenses
Concave lenses are often used in the
viewfinders of cameras. Concave lenses are
also combined with mirrors or other lenses to
form images in optical instruments such as
telescopes.
19.2 Lenses
Concave and Convex Lenses
Convex Lenses
A convex lens is curved outward at the center
and is thinnest at the outer edges.
• As the rays pass through the lens, each one is
refracted, and they emerge at different angles.
• Convex lenses cause incoming parallel rays to come
together, or converge.
19.2 Lenses
Concave and Convex Lenses
• The converging rays meet at the focal point, on
the side of the lens opposite to the object.
• Whether an image is real or virtual depends
upon how far the object is from the lens.
19.2 Lenses
Concave and Convex Lenses
Each of the housefly’s
eyes is made up of
thousands of tiny
individual eyes called
facets.
The outer surface of
each facet is convex.
The eyes give the fly a
nearly 360-degree
field of view.
19.2 Lenses
Concave and Convex Lenses
A. Parallel rays are refracted and pass through
the focal point of a convex lens.
Focal
point
Optical axis
Convex lens
19.2 Lenses
Concave and Convex Lenses
B. When an object is located beyond the focal
point, a real image is formed.
Focal
point
Object
Focal
point
Convex lens
Real
image
19.2 Lenses
Concave and Convex Lenses
C. A magnified, virtual image is formed when the
object is located between the focal point and
the lens.
Virtual image
Focal
point Object
Convex lens
Focal
point
Focal
point
19.2 Lenses
Concave and Convex Lenses
Convex lenses are used in slide and movie
projectors and cameras.
At the movie theatre, the film is placed upside
down in the projector so that the real image is
upright.
19.2 Lenses
Concave and Convex Lenses
In the past, lighthouses
used a light source
placed at the focal point
of a convex lens or
series of convex lenses
to form a beam of
parallel light rays.
19.2 Lenses
Total Internal Reflection
In what types of materials is total internal
reflection likely to occur?
Materials that have small critical angles are
likely to cause most of the light entering
them to be totally internally reflected.
19.2 Lenses
Total Internal Reflection
The critical angle is the angle of incidence that
produces an angle of refraction of 90 degrees.
• At the critical angle, the light ray bends so much
that it takes a path along the glass-air boundary.
• At angles larger than the critical angle, the light ray
bends so much that it is reflected back into the
glass.
19.2 Lenses
Total Internal Reflection
Total internal reflection is the complete
reflection of a light ray back into its original
medium. An important application of total internal
refraction is fiber optics.
• Light rays are generally unable to exit through the
sides of the curving fiber optic strands.
• Fiber optics are able to transmit data in the form of
light pulses over large distances with little loss in
signal strength.
19.2 Lenses
Total Internal Reflection
A.
For angles less than the critical angle, light is partly refracted
and partly reflected.
19.2 Lenses
Total Internal Reflection
A.
B.
For angles less than the critical angle, light is partly refracted
and partly reflected.
At the critical angle, the angle of refraction is 90 degrees.
19.2 Lenses
Total Internal Reflection
A.
B.
C.
For angles less than the critical angle, light is partly refracted
and partly reflected.
At the critical angle, the angle of refraction is 90 degrees.
For angles greater than the critical angle, all of the light is
reflected.
19.2 Lenses
Assessment Questions
1. Light refracts when it passes from air to water
because
a.
b.
c.
d.
the wavelength is different in the two media.
the frequency is different in the two media.
the speed is different in the two media.
the amplitude is different in the two media.
19.2 Lenses
Assessment Questions
1. Light refracts when it passes from air to water
because
a.
b.
c.
d.
the wavelength is different in the two media.
the frequency is different in the two media.
the speed is different in the two media.
the amplitude is different in the two media.
ANS: C
19.2 Lenses
Assessment Questions
2. Which type(s) of lens can form a real image?
a.
b.
c.
d.
concave lens only
convex lens only
both concave and convex lenses
neither concave nor convex lenses
19.2 Lenses
Assessment Questions
2. Which type(s) of lens can form a real image?
a.
b.
c.
d.
concave lens only
convex lens only
both concave and convex lenses
neither concave nor convex lenses
ANS: B
19.2 Lenses
Assessment Questions
3. What will happen to a ray of light if it hits a new
medium at an angle greater than the critical
angle?
a.
b.
c.
d.
All the light will be refracted.
Part of the light will be refracted and part reflected.
All the light will be reflected.
All the light will be absorbed.
19.2 Lenses
Assessment Questions
3. What will happen to a ray of light if it hits a new
medium at an angle greater than the critical
angle?
a.
b.
c.
d.
All the light will be refracted.
Part of the light will be refracted and part reflected.
All the light will be reflected.
All the light will be absorbed.
ANS: C
19.2 Lenses
Assessment Questions
1. The index of refraction is the ratio of the speed of
light in a vacuum to the speed of light in a
material.
True
False
19.2 Lenses
Assessment Questions
1. The index of refraction is the ratio of the speed of
light in a vacuum to the speed of light in a
material.
True
False
ANS:
T